fish
鮆 centers on fish as a general category. The single given meaning is simply 'fish', so the kanji refers to fish in a broad sense.
鮆 combines 魚 (fish) with 此, which likely serves as a phonetic component. The exact historical development is uncertain, but the structure suggests a phono-semantic compound where 此 indicates the sound and 魚 specifies the meaning category.
The bottom is 魚 (fish), and the top 此 looks like a person stopping to compare something. Imagine a fishmonger comparing fish at the market: 鮆 is all about fish.
For セイ, picture a fish saying 'say' as it leaps out of the water: say -> セイ, and the fish speaks its own name.